Carbon and graphite electrodes are often coated with various substances to enhance their properties. For example, oxidation-retardant substances are impregnated into electrodes or applied to electrode surfaces to inhibit oxidation during use of the electrode. Such an oxidation-retardant system is disclosed in U.S. Pat. No. 4,726,995. Generally, application of coating materials to electrode surfaces involves application of a precursor material to the surface of the electrode and then heating the electrode to transform the precursor material into the final protective material. For example, in the process of U.S. Pat. No. 4,726,995, the electrode is impregnated with a liquid composition comprising a phosphate compound, a halide-containing compound and a solvent for the halide containing compound such as water. The entire electrode body is then heated to a treat temperature of between 500.degree. C. and 600.degree. C., in a conventional gas oven for a period of between 1 and 3 hours, to convert the impregnate into an insoluble phosphate compound.
The prior art coating method of impregnating the entire electrode body with a coating precursor, and then heating the entire electrode body to the required temperature for converting the precursor into the final protective coating is expensive, time consuming and above all wasteful of impregnate material. U.S. Pat. No. 4,726,995 also teaches rolling the electrode in a bath to impregnate the electrode to a limited depth before heating in order to provide savings in the quantity of coating solution applied. However, when the electrode is heated, the entire electrode body is heated, even that portion not impregnated with coating solution, resulting in a significant waste of thermal energy to heat unnecessary portions of the electrode.
In addition, the prior art method does not lend itself to automation and requires a significant investment in time to cure each electrode separately in a batch type operation. As discussed above, the entire electrode mass is heated in order to heat the impregnate in the electrode to its thermal conversion temperature (hereinafter referred to as the "treat" temperature). Heating the entire electrode to the treat temperature requires several hours. Furthermore, the gas ovens typically used for heating electrodes require an extensive investment in time to load and unload the furnace. Following conversion of the precursor material, the electrode must then be cooled to ambient temperature so as to permit handling and stacking. This may again involve several hours.
The end result is that the prior art technique is both time and energy intensive, which translates into excessive energy costs, excessive capital equipment requirements, and increased inventory overhead.